Magnesium-based alloy and method for the production thereof

ABSTRACT

The invention relates to magnesium-based alloys and, more specifically, to a magnesium alloy composition and methods of producing the same. The alloys have improved mechanical properties in that creep ratios are decreased. The magnesium-based alloys comprise aluminium, zinc, manganese and silicon. A method for producing said alloy consists of loading the alloying components, pouring the molten magnesium, introducing a titanium-containing fusion cake with a flux agent and continuously agitating. The alloy is then soaked and cast.

FIELD OF THE INVENTION

This invention relates generally to magnesium-based alloys and morespecifically to magnesium alloy composition and methods of producingthem that are widely used in the automotive industry.

BACKGROUND OF THE INVENTION

There are various alloys developed for special applications including,for example, die casting of automotive components. Among these alloysmagnesium-aluminium alloys can be designated as cost-effective andwidely used for manufacture of automotive parts, e.g. AM50A alloy (whereAM means aluminium and manganese are in the composition of the alloy)containing approx. 5 to 6 wt. % aluminium and manganese traces, andmagnesium-aluminium-zinc alloys, e.g. AZ91D (where AZ means aluminiumand zinc are in the composition of the alloy) containing approx. 9 wt. %aluminium and 1 wt. % zinc.

The disadvantage of these alloys is their low strength and poor creepresistance at elevated operating temperatures. As a results, the abovementioned magnesium alloys are less suitable for motor engines wheresome components such as transmission cases are exposed to temperaturesup to 150° C. Poor creep resistance of these components can lead to adecrease in fastener clamp load in bolted joints and, hence, to oilleakage.

Known in the present state of art is a magnesium-based alloy (Inventors'certificate No. 442225 issued in Invention Bulletin 33, 1974) containingaluminium, zinc, manganese, silicium as alloying components in thefollowing contents:

-   -   Aluminium—6-15 wt. %    -   Zinc—0.3-3.0 wt. %    -   Manganese—0.1-0.5 wt. %    -   Silicium—0.6-2.5 wt. %    -   Magnesium—rest being

The disadvantages of this alloy are its low ductility, high hotshortness, and insufficient strength of the alloy which keeps this alloyfrom automotive applications.

Known presently is another magnesium die cast alloy (“Magnesium alloys”in Collected works of Baikov Institute for Metallurgy edited by NaukaPublishing House, 1978, p.140-144) which comprises aluminium, zinc,manganese, silicium as alloying components in the following contents:

-   -   Aluminium—3.5-5.0 wt. %    -   Zinc—under 0.12 wt. %    -   Manganese—0.20-0.50 wt. %    -   Silicium—0.5-1.5 wt. %    -   Copper—under 0.06    -   Nickel—0.03 wt. %

The drawback of this alloy is that the quantitative composition of thealloy selected provides poor mechanical properties, in particular, thealloy having a small solidification range is characterised with advancedsusceptibility to cracking in case of hindered contraction and badcastability.

A well-known German standard EN 1753-1997 is taken as the closest priorart by its qualitative and quantitative composition and discloses themethods of manufacture of EN MB MgAl2Si and EN MB MgAl4Si alloys. Thequalitative analysis of the alloys is the following, in wt. %:

-   -   EN MB MgAl2Si:    -   Al—1.9-2.5    -   Mn—min 0.2    -   Zn—0.15-0.25    -   Si—0.7-1.2    -   EN MB MgAl4Si (AS41):    -   Al—3.7-4.8    -   Mn—0.35-0.6    -   Zn—max 0.10    -   Si—0.6-1.4

The alloys of the above quantitative and qualitative compositiondemonstrate better mechanical properties. However, at 150-250° C. thesealloys have high creep that keeps these alloys from machine-buildingapplication. Presently known is the method (PCT Patent No. 94/09168) formaking a magnesium-based alloy that provides for alloying components ina molten state being introduced into molten magnesium. Primary magnesiumand alloying components are therefor heated and melted in separatecrucibles.

What is disadvantageous of this method is the need to pre-melt manganeseand other alloying elements (at the melting temperature of 1250° C.)that complicates alloy production and process instrumentation.

There are some other methods known (B. I. Bondarev “Melting and Castingof Wrought Magnesium Alloys” edited by Metallurgy Publishing House,Moscow, Russia 1973, pp 119-122) to introduce alloying components usinga master alloy, e.g. a magnesium-manganese master alloy (at the alloyingtemperature of 740-760° C.).

This method is disadvantageous because the alloying temperature shouldbe kept high enough which leads to extremely high electric powerconsumption for metal heating and significant melting loss.

Also known is another method of producing amagnesium-aluminium-zinc-manganese alloy (I. P. Vyatkin, V. A. Kechin,S. V. Mushkov in “Primary magnesium refining and melting” edited byMetallurgy Publishing House, Moscow, Russia 1974, pp. 54-56, pp. 82-93)which is taken as an analogue-prototype. This method stipulates variousways how to feed molten magnesium, alloying components such asaluminium, zinc, manganese. One of these approaches includessimultaneous charging of solid aluminium and zinc into a crucible, thenheating above 100° C., pouring in molten primary magnesium and againheating up to 700-710° C. and introducing titanium-containing fusioncake together and manganese metal under continuous agitation.

The main shortcoming of the method is in considerable loss of alloyingcomponents resulting in lower recovery of alloying components inmagnesium and preventing from producing alloys with specified mechanicalproperties. Furthermore, this increases the cost of the alloy.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to improvemechanical properties of the alloy and, in particular, to decrease itscreep and loss of alloying constituents in manufacturing the alloy.

Said invention makes it possible to reduce the production costs of thealloy and to improve the performance characteristics thereof in order toextend the use of said alloy for the automobile industry.

These objects are accomplished due to the fact that the claimedmagnesium-based alloy comprises aluminium, zinc, manganese and silicium,wherein the constituents specified are in the following components, wt.%:

-   -   Aluminium—2.5-3.4    -   Zinc—0.11-0.25    -   Manganese—0.24-0.34    -   Silicium—0.8-1.1    -   Magnesium—rest being

To manufacture the alloy there is a method for producing which consistsin loading of alloying components, pouring of molten magnesium,introducing a titanium-containing fusion cake together with a flux agentand continuous agitation, and the alloy is soaked and casted, wherein inloading alloying components of aluminium, zinc, manganese, and siliciumin the form of a ready-made solid master alloy ofaluminium-zinc-manganese-silicium master alloy, after poured in themagnesium is heated, subjected to ageing and then stirred.

Further, the proportion of the master alloy to magnesium is 1:(18-20).

Further, magnesium is heated up to 720-740° C.

Further, the ageing process lasts for 1-1.5 hrs.

Said quantitative composition of the magnesium-based alloy enablesbetter mechanical properties of the alloy.

Aluminium added into magnesium contributes to its tensile strength atambient temperature and alloy castability. However, it is well-knownthat aluminium is detrimental to creep resistance and strength ofmagnesium alloys at elevated temperatures. This results from the casethat aluminium, when in higher contents, tends to combine with magnesiumto form great amounts of intermetallic Mg₁₇Al₁₂ having a low meltingtemperature (437° C.) which impairs high-temperature properties ofaluminium-based alloys. Aluminium content of 2.5-3.4 wt. % that waschosen for the proposed magnesium-based alloy provide better propertiesof magnesium-based alloys, such as creep resistance.

The properties of the alloy, especially its castability, are furtherinfluenced by zinc content; however, added in large amounts, zinc canlead to cracking. Therefore, proposed zinc content is within 0.11-0.25wt. % to be optimum for the magnesium-based alloy.

In order to enhance service performance and functionality and expand thescope of application at higher temperatures (up to 150-200° C.) siliconis added into the alloy as an active alloying additive to form ametallurgic stable phase Mg₂Si precipitated slightly at grain boundariesand, hence, to increase creep resistance of the alloy at hightemperatures. Silicon content of 0.8-1.1 wt. % claimed in accordancewith the present invention enables decreasing creep level of themagnesium-based alloy.

The alloy is loaded with manganese in the content 0.24-0.34 wt. % inorder to ensure corrosion resistance.

The alloying componentsts are introduced in the form of the pre-preparedaluminium-zinc-manganese-silicon master alloy, which is added in thecertain proportion to magnesium, i.e. 1:(18-20), and this, therefore,enhances recovery of the additives in magnesium, thus lowering losses ofexpensive chemicals.

It is another difficulty in making alloys with silicon content thatsilicium and manganese as alloying components come to a reaction formingheavy intermetallic phases Mn₃Si and MnSi₂, which deposit at the bottomof crucibles at the end of production process, and this hinders highlevel of recovery of these components. Thus, a better recovery of thealloying additives can be produced using the pre-preparedaluminium-based master alloy.

With process temperature maintained at 720-740° C. the level of recoveryof alloying elements in magnesium can be 98.8-100% in case of aluminium,68.2-71.1% in case of manganese, 89.3-97.4 in case of silicon,85.9-94.4% in case of zinc.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Preparation of Al—Mn—Si—Zn Master Alloy

Composition: aluminium—matrix, manganese—6.0-9.0 wt. %,silicium—24.0-28.0 wt. %, zinc—2.0-3.0 wt. %, inclusions, in wt. %:iron—0.4, nickel—0.005, copper—0.1, titanium—0.1. The master alloy isproduced in ingots.

The master alloy is manufactured in an ‘AIAX’-type induction furnace.A97 grade aluminium (acc. to GOST 11069) is charged in the furnace,heated up to 910-950° C.; the master alloy is melted under cryolite fluxin the amount of 1-1.5% of the pre-weighted quantity required for theprocess. Kpl (Kr1) grade crystalline silicon is fed in portions in theform of crushed pieces, it is a possible means that the pieces ofsilicium be wrapped in aluminium foil or wetted with zinc chloridesolution to prevent them from oxidation. Silicium is dissolved in smallportions being thoroughly stirred. The composition obtained isthereafter added with manganese metal of MH95 grade (Mn95 acc. to GOST6008) in the form of 100 mm pieces, stirred again and heated up to thetemperature within 800-850° C.; finally added with

1-grade zinc (Z1 acc. to GOST 3640). 16 kg ingots are cast in moulds.

EXAMPLE 1

Solid master alloy of Al—Mn—Si—Zn in the form of ingots in theproportion of master alloy to magnesium 1 :(18-20) are charged into apreheated crucible of furnace SMT-2, in the same crucible raw magnesiumMΓ90 (MG-90 acc. to GOST 804-93) is poured in the amount of 1.8 tonsfrom a vacuum ladle and is afterwards heated. On reach 730-740° C. ofthe metal temperature a heated agitator is placed in the crucible, thealloy is left undisturbed in the crucible for 1-1.5 hrs prior to mixingand then mixed for max. 40-50 min; introduced a titanium-containingfusion cake (TU 39-008) being in the compound with barium flux in theproportion of 1:1 is added, mixed again; the temperature of the alloy isthen reduced to 710-720° C., the alloy produced was left staying in thecrucible for 60 min and thereafter the alloy was sampled for thecomplete chemical analysis to define Al, Mn, Zn, Si contents andimpurities. The alloy composition in wt. %: Al—2.5-3.4, Mn—min 0.23,Si—0.8-1.3, Be—0.0008-0.0012, Zn—min 0.18, Fe—min 0.003.

Industrial Applicability TABLE 1 Mechanical properties of themagnesium-based alloy at 150° C. Creep test Mechanical Creep ratioproperties at Type of alloy σ, MPa σ, % 150° C., σ_(B) MPa AZ91 45.00.82 136 EN MB MgAl₂Si 45.0 0.490 128 (AS 21) EN MB MgAl₄Si 45.0 0.540139 AS 31 alloy claimed 45.0 0.143 128

TABLE 2 Level of recovery of alloying elements in magnesium ConstituentsRecovery level, % Aluminium 100 Manganese 73.5-96.3; at 720-740° C. andtime of agitation 40-50 min recovery level of manganese is 80-96%Silicon 80.8-92.5 Zinc 84.8FIGS. 1 and 2 illustrates the level of recovery of alloying elements inmagnesium depending on the temperature and time of agitation.

Thus, the magnesium-based alloy of said qualitative composition and themethod to prepare it facilitate improving mechanical properties of thealloy, particularly, to decrease creep by 3-4 times, reduce productioncosts due to a better recovery of alloying components in magnesium.

1. A magnesium-based alloy containing aluminium, zinc, manganese andsilicon, wherein the constituents specified are in the followingcomponents, wt. %: Aluminium-2.5-3.4 Zinc-0.11-0.25 Manganese-0.24-0.34Silicon-0.8-1.1 Magnesium-remainder.
 2. A method for producing amagnesium-based alloy comprising the steps of: a. loading alloyingcomponents in the form of a master alloy including i. Al ii. Zn iii. Siiv. Mn; b. pouring molten Mg i. heating ii. aging; and iii. stirring; c.introducing a Ti-containing fusion cake with a fluxing agent i.continuously agitating the cake to form an intermediate composition; d.cooling the intermediate composition; e. soaking; and f. casting.
 3. Themethod of claim 2, wherein the proportion of the master alloy content tomagnesium is 1:(18-20).
 4. The method of claim 2, wherein magnesium isheated up to 720-740° C.
 5. The method of claim 2, wherein the ageing iscarried out within 1-1.5 hrs.
 6. The method of claim 2 wherein theloading step comprises loading components in the form of a ready-madesolid master alloy.
 7. The method of claim 2 wherein the step of loadingalloying components further comprises providing Al in the amount of2.5-3.4 wt. %.
 8. The method of claim 2 wherein the step of loadingalloying components comprises providing Si in the amount of 0.8-1.1 wt.%, the Si forming a metallurgically stable phase of Mg₂Si that isprecipitated at grain boundaries, thereby improving the mechanicalproperties of the alloy.
 9. The method of claim 2 wherein the step ofloading alloying components further comprises providing Zn in the amountof 0.11-0.25 wt. % for fluidity.
 10. The method of claim 2 wherein thestep of loading alloying components further comprises the step ofproviding Mn in the amount of 0.24-0.34 wt. % for corrosion resistance.11. The method of claim 2 wherein the step of loading alloyingcomponents in the form of a master alloy comprises adding a ready-madesolid master alloy in a proportion to Mg of 1:(18-20) for the recoveryof additives and reducing the loss of chemicals.
 12. The method of claim2 wherein steps (a-c) are conducted at 720-740° C., thereby enabling alevel of alloy recovery of Al (98.8-100%); Mn (68.2-71.1%); Si(89.3-97.4%); and Zn (85.9-94.4%).
 13. The method of claim 2 wherein themaster alloy comprises the following components, wt. %: Mn—6.0-9.0Si—24.0-28.0 Zn—2.0-3.0 an inclusion selected from a group consisting ofFe 0.4; Ni 0.005; Cu 0.1; Ti 0.1 and combinations thereof; andAl—remainder.